Titania is a white pigment, available in rutile and anatase crystalline forms, and is widely used to impart opacity in paper, paints and plastics. Calcined kaolin clay, especially low abrasion forms thereof, is frequently used alone or in combination with titania when opacification is desired at a lower cost. It is well known that calcination of kaolin results in increased light scattering of the clay, manifest by enhanced brightness and opacification of coated and filled paper and paint films. Thus, calcined kaolin pigments such as the product available commercially from Engelhard Corporation under the registered trademarks ANSILEX and ANSILEX 93, are widely used by the paper industry and the paint industry. Heating of kaolin particles results in a thermally bulked structure. See, for example, U.S. Pat. No. 3,586,523, Fanselow et al. The conversion of kaolin clay to calcined clay may be represented by the equation: ##STR1## This is an oversimplification of the reaction involved in the transformation because the thermal treatment effects profound changes in the morphology, particle size distribution and light scattering properties of the clay. It also changes the chemical reactivity of the clay in both acidic and alkaline media.
Calcined kaolin clay is frequently used as an extender for titania pigments in paper coating and filling. See, for example, U.S. Pat. No. 4,874,466, Savino in which a cationic polymer is used as a dispersant for the titania and paper is filled with this titania using a filler formulation that also includes calcined kaolin. It has already been proposed to provide composite titania pigments which provide optimum spacing for the titania particles to maximize opacification and other properties. Reference is made to U.S. Pat. No. 3,726,700, Wildt. In accordance with the teachings of this patent, titania particles are adhered to particles of clay, preferably calcined kaolin, by a precipitate of inorganic oxides (silica, titania, alumina and combinations thereof), the oxides being formed by precipitation of alkaline or acidic precursor salts with appropriate pH adjustment of a slurry containing a mixture of the titania and calcined kaolin particles. In illustrative examples, the composite pigment product contained 42% TiO.sub.2, the balance being calcined kaolin and inorganic oxide cement. Recently a titania-calcined kaolin composite pigment which appears to contain an organic adhesive and a similar level of TiO.sub.2 has been introduced to the paper industry.
Successful commercialization of composite titania-calcined kaolin composite pigments, however, requires more than the provision of products which are highly effective opacifying agents. In most cases, such as in paper applications, desirable opacifying pigments should have acceptable fluidity when dispersed in water at practical pigment solids levels and these slurries must resist settling under static condition. For example, the pigment should be capable of being formed into a high solids slurry e.g., one containing at least 50% pigment solids, more preferably at least 52% pigment solids and, most preferably, at least 55% pigment solids. Such slurry should be sufficiently fluid to have a Brookfield viscosity (20 rpm) below 1000 cp, preferably below 500 cp at 20 rpm. See U.S. Pat. No. 4,772,332, Nemeh et al. Not only must the slurry be fluid, but it must resist settling. The inability to provide composites satisfying the dual criteria of rheology and efficient scattering may explain the long interval between the earlier proposal to enhance the efficiency of titania pigments by adhering them to particles of calcined kaolin and present day limited commercial use of such composite pigments.
In addition to the thermal means (calcination) heretofore employed to bulk kaolin clay, chemical means have been used to bulk kaolin clay but these means have been restricted to kaolin in uncalcined (hydrous or hydrated) form. A commercial product available from Engelhard Corporation is supplied under the registered trademark EXSILON and is produced by using a cationic polymer to bulk hydrated kaolin. See, for example, U.S. Pat. No. 4,738,426, Pratt et al. Cationic polymers of this type are also known to be pigment dispersants when employed at adequate levels, generally substantially in excess of levels needed to bulk anionic pigments such as hydrous kaolin.
Observations of the desirable effects of bulking hydrous kaolin with cationic polymer lead to the expectation that addition of cationic polymer to a calcined kaolin pigment would also enhance the scattering of the calcined kaolin pigment, resulting in a premium grade of calcined kaolin pigment. Surprisingly, the attempts were not successful. Similarly, the expectation was that addition of cationic polymer to a titania pigment in the manner used to bulk hydrous kaolin would lead to benefits such as those achieved with the hydrous kaolin. Surprisingly, these attempts were also not successful.